1. Field of the Invention
The present invention relates to a controlling method of a control device controlling data input and output to a plurality of storage devices, and a control device used in the method, and particularly relates to a disk array device, and a controlling method thereof.
2. Description of the Related Art
Recently, there have been an increasing number of situations which require large amounts of data to be stored, such as meteorological information or digital broadcasts for the transmission of high-definition television. As a result, larger capacities are required for external storage devices that are used to store this data. One method of reliably storing large amounts of data is by using a disk array device referred to as RAID (Redundant Arrays of Inexpensive Disks) which is equipped with a plurality of magnetic disks. Thus, efforts have been made to increase data processability by dispersing and storing data in a plurality of magnetic disks and to increase reliability by storing redundant data.
An example of a RAID disk array device is disclosed in the paper, “A Case for Redundant Arrays of Inexpensive Disks (RAID)”, by David A. Patterson, Garth Gibson, and Randy H Katz of the Computer Science Division of the Electrical Engineering and Computer Sciences Department at the University of California, Berkeley.
As described above, this disk array device has a larger capacity, higher conductance, and greater reliability as compared to a device with a single magnetic disk. Accordingly, these disk array devices are now being employed in such diverse settings such as television stations, banks, hospitals, research laboratories, and universities.
As is well known, since parity calculation and the like are necessary at the time of the initial installation of the disk array device, a process of writing initialization data to all storage regions in the disk unit (storage device), namely a logic format, is necessary.
There are two methods of logic format which are used: simple format (or quick format) and format with data-verifying function.
In simple format, the portion corresponding to data in the object to be formatted is not examined, and only the portion corresponding to the header is rewritten. As an example of using simple format, a technique disclosed in Japanese Patent Application Laid-open No. Hei 9-330180 is referred to. With the technique described in this publication, a generation state of parity is managed for each stripe group in the RAID disk array, and by determining the algorithm of parity calculation when data is written according to the state, the requirement for performing a logic format is abolished at initial installation.
Although simple format has the advantage of completing the format instantly, its reliability is low and an error can possibly occur in data that the user has written. In particular, if there is an error such as a letter encoding or decoding error, the data cannot be recovered.
In order to avoid such problems, it is necessary to verify data at the time of format. There are three methods of format with data-verifying function (a), (b), and (c) which can be utilized in response to a format request from a host device: (a) an initial value of data is written in the data storing area to be formatted; (b) when a RAID composition is employed, redundant data calculated from initial value data is written in all of redundant data storing regions to be formatted; and (c) both (a) and (b) are performed.
When format with data-verifying function is employed in an external storing device (including a disk array device) requiring reliability, a command to the magnetic disk devices is given, which requires a format process time proportional to the size of a region to be formatted to complete.
An improved technique of such a format process is disclosed in the Japanese Patent Application Laid-open No. Hei 3-4315, where firmware in a sub-system is provided with a format function in respect to a format request from a host device. Thus, the device may conduct formatting in the subsystem independently from the host device.
A format for securing reliability in this way is carried out primarily when the external storage device is initially installed, or when the number of magnetic disks are increased and the amount of data regions for users is newly increased.
When a disk array device is initially installed and a format with data-verifying function is performed, all areas used for storing data that are mounted to a magnetic disk device must be formatted at once. Thus, there arises a problem where a format time proportional to the increase of storage capacity is required.
In recent systems, storage capacity has been increasing in large amounts, which usually requires several days for the entire format. Furthermore, when increasing the number of magnetic disks used, it is necessary to format all newly added areas for storing data, which requires a format time proportional to the increase of storage capacity. Thus, when a format process is occurring, the system cannot respond to data input and output orders from external devices.
In regards to a storage system whose capacity is assumed to continually increase, simple format has an advantage in that formatting is completed instantaneously, but it has a disadvantage in that reliability decreases, as described previously. On the other hand, format with data-verifying function has an advantage in that reliability is high, but the disadvantage is that time taken until the format is complete is extremely long.
With the technique disclosed in Japanese Patent Application Laid-open No. Hei 10-3358, in response to a format request from a host device, completion of a format process is instantly reported, and that format process is performed in a subsystem. However, in this technique, the subsystem that is in the process of formatting cannot receive and process I/O requests from the host computer.